Right now, without VoIP API, if you get a cellular phone call while using a VoIP app, your call gets put on hold/dropped/whatever because the cellular call takes precedence. What will the behavior be like after apps update using the VoIP API?
I have Skype for Business installed on my phone and while I see FaceTime and regular telephony as options for contacts, I do not see Skype. Should I?
You also get the green banner at the top to return to the call when you go out of the call with the home button and so on.This one: https://developer.apple.com/videos/play/wwdc2016/230/
I can summarise it like this: VoIP calls will be visually and functionality indistinguishable from regular calls and FaceTime calls. They use the same UI and adopt most of the same behaviour. When you receive an actual call while you are VoIPing using this API, then you can deal with it in the same way: switch to the call or put in on hold. They are also integrated in the phone app and contacts, Siri, Bluetooth and the do-not-disturb mode.